#include <stdio.h>

/**
 * Project Euler Problem 11
 *
 * In the 20×20 grid below, four numbers along a diagonal line have been marked in red.
 *
 * (Assume starting at row 06, column 08 in provided grid array in source code)
 *
 * The product of these numbers is 26 × 63 × 78 × 14 = 1788696.
 * 
 * What is the greatest product of four adjacent numbers in the same direction
 * (up, down, left, right, or diagonally) in the 20×20 grid?
 */
int main() {
    int grid[20][20] =
        {
            { 8,  2, 22, 97, 38, 15,  0, 40,  0, 75,  4,  5,  7, 78, 52, 12, 50, 77, 91,  8},
            {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48,  4, 56, 62,  0},
            {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30,  3, 49, 13, 36, 65},
            {52, 70, 95, 23,  4, 60, 11, 42, 69, 24, 68, 56,  1, 32, 56, 71, 37,  2, 36, 91},
            {22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80},
            {24, 47, 32, 60, 99,  3, 45,  2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50},
            {32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70},
            {67, 26, 20, 68,  2, 62, 12, 20, 95, 63, 94, 39, 63,  8, 40, 91, 66, 49, 94, 21},
            {24, 55, 58,  5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
            {21, 36, 23,  9, 75,  0, 76, 44, 20, 45, 35, 14,  0, 61, 33, 97, 34, 31, 33, 95},
            {78, 17, 53, 28, 22, 75, 31, 67, 15, 94,  3, 80,  4, 62, 16, 14,  9, 53, 56, 92},
            {16, 39,  5, 42, 96, 35, 31, 47, 55, 58, 88, 24,  0, 17, 54, 24, 36, 29, 85, 57},
            {86, 56,  0, 48, 35, 71, 89,  7,  5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
            {19, 80, 81, 68,  5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77,  4, 89, 55, 40},
            { 4, 52,  8, 83, 97, 35, 99, 16,  7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
            {88, 36, 68, 87, 57, 62, 20, 72,  3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
            { 4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18,  8, 46, 29, 32, 40, 62, 76, 36},
            {20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74,  4, 36, 16},
            {20, 73, 35, 29, 78, 31, 90,  1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57,  5, 54},
            { 1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52,  1, 89, 19, 67, 48}
        };

    int prod = 0;
    for (int i = 0; i < 20; i++) {
        for (int j = 0; j < 20; j++) {
            int vertical = 0, horizontal = 0, diagonal_left = 0, diagonal_right = 0;
            if (j < 17) {
                horizontal = grid[i][j] *
                       grid[i][j + 1] *
                       grid[i][j + 2] *
                       grid[i][j + 3];
            }
            if (i < 17) {
                vertical = grid[i][j] *
                       grid[i + 1][j] *
                       grid[i + 2][j] *
                       grid[i + 3][j];
            }
            if (i < 17 && j < 17) {
                diagonal_right = grid[i][j] *
                        grid[i + 1][j + 1] *
                        grid[i + 2][j + 2] *
                        grid[i + 3][j + 3];
            }
            if (i > 3 && i < 17 && j > 3) {
                diagonal_left = grid[i][j] *
                        grid[i + 1][j - 1] *
                        grid[i + 2][j - 2] *
                        grid[i + 3][j - 3];
            }

            if(horizontal > prod){ prod = horizontal;}
            if(vertical > prod){prod = vertical;}
            if(diagonal_left>prod){prod = diagonal_left;}
            if(diagonal_right>prod){prod = diagonal_right;}
        }
    }

    printf("The highest product of 4 contiguous numbers in the grid is %d\n", prod);
    return 0;
}